1. Field of the Invention
The present invention relates to an enclosed lead storage battery which employs, as electrode plates, grids each having no frames at its opposite lateral sides, such as an expanded grid.
2. Description of the Related Art
Generally, a group of electrode plates for use in an enclosed lead storage battery are constructed by layering a positive electrode plate, a separator formed of a water-absorbent glass mat, and a negative electrode plate in sequence side by side, and are then housed in an enclosed container. When an electrolyte is poured into the container housing the group of electrode plates therein, all of the electrolyte is held in the water-absorbent glass mat serving as the separator. Therefore, the electrolyte is hardly present in the container under such a free or liberated condition that the electrolyte is too much in excess of the electrolyte holding ability of the separator. In such an enclosed lead storage battery, oxygen generated from the positive electrode plate in an overcharged state is absorbed by the negative electrode plate, thereby preventing an excessive rise in internal pressure of the storage battery.
In many cases, cast grids each having frames on its opposite lateral sides are employed as grids constituting the positive and negative electrode plates. Also, as stated above, the electrolyte is kept from being present under the free or liberated condition. Therefore, lead is prevented from depositing on the opposite lateral sides of the electrode plate, thereby ensuring the protection against a short-circuit.
Meanwhile, grids each having no frames on its opposite lateral sides (e.g., expanded grids) have often been employed as, particularly, the negative electrode plates in recent years. For an electrode plate employing such an expanded grid which has no frames on its opposite lateral sides, however, there is a fear that the repetition of charging and discharging cycles may cause an active material to swell out of the opposite lateral sides of the negative electrode plate, extend further along the inner wall surfaces of the container beyond the separator, and finally come into contact with the opposite lateral sides of the positive electrode plate, thus giving rise to a short-circuit.